The present invention relates to corrosion inhibition, and more particularly to inhibition of corrosion in environmentally sensitive aqueous media.
Corrosion of metal surfaces in aqueous media, such as sea water, is a longstanding problem. The problem is especially troublesome in deep sea operations, such as offshore drilling and production, where conditions are particularly rigorous. Corrosion inhibitors for use in offshore operations must be effective under demanding deep sea conditions, and also must be environmentally acceptable. The corrosion inhibitors must meet stringent standard toxicity requirements, and also should be compatible with the sensitive life forms that are indigenous to the area. For example, in North Sea operations, the corrosion inhibitor should be compatible not only with fish, but also with indigenous algae, such as Skeletonema costatum. 
Commonly used inhibitors have proven to be too toxic for compatibility with Skeletonema costatum. Even a concentration of less than one part per million by weight (ppm) of conventional inhibitors has been found to retard growth of Skeletonema costatum test populations by 50% in 96 hours (EC50 less than 1 ppm). Corrosion inhibitors are needed which have an EC50 greater than 1 ppm for Skeletonema costatum. 
The corrosion inhibitor also should be sufficiently biodegradable that, within 28 days after treatment, the inhibitor degrades at least 60%, most preferably 100% in terms of the theoretical oxygen consumption required for complete degradation (i.e., the biochemical oxygen demand BODxe2x88x9228xe2x89xa760%, preferably=100%). The inhibitor also should be sufficiently water soluble to avoid or minimize bio-accumulation in fat in lower life forms. Fat soluble inhibitors tend to become more concentrated as they move up the food chain.
Imidazolines have promise as corrosion inhibitors from an environmental standpoint because imidazolines are effective as corrosion inhibitors even though they do not contain sulfur or phosphorus. However, imidazoline inhibitors are needed which are both effective as corrosion inhibitors and which also meet stringent toxicity standards, such as an EC50 greater than 1 ppm for Skeletonema costatum. 
The present invention provides a method of inhibiting corrosion of metal equipment in an aqueous medium comprising components selected from the group consisting of Skeletonema costatum, fish, other algae, and a combination thereof, said method comprising incorporating into the aqueous medium an amount of a water soluble corrosion inhibitor effective to inhibit said corrosion. The corrosion inhibitor comprises an N-ethoxy, 2-substituted imidazoline. The N-ethoxy substituent comprises a quantity of ethylene oxide effective to render the imidazoline water soluble. The 2-substituent comprises a fatty acid chain consisting essentially of 18 carbon atoms or less.
The present invention provides imidazolines with reduced toxicity which are effective to inhibit the corrosion of metal equipment in an aqueous environment. Toxicity is minimized by reducing the chain length of the acid used to make the imidazoline. Imidazolines with shorter chain lengths tend to be less effective as corrosion inhibitors; however, the addition of certain wetting agents has been found to increase the effectiveness of these less toxic imidazolines as corrosion inhibitors.
Preferred corrosion inhibitors do not contain sulfur or phosphorus and are xe2x80x9cenvironmentally compatible.xe2x80x9d As used herein, the term environmentally compatible shall mean that a substance has little or no deleterious environmental effects on a medium of concern, and includes, but is not necessarily limited to considerations such as toxicity, water-solubility, biodegradability, and so forth. Although the term xe2x80x9cnon-toxicxe2x80x9d is used herein, nearly every substance is toxic at some concentration. The term xe2x80x9cnon-toxicityxe2x80x9d refers to very low toxicity at the relevant concentration. For example, for offshore drilling and production, the term xe2x80x9cnon-toxicityxe2x80x9d or xe2x80x9cnon-toxicxe2x80x9d refers to compositions having and EC50 greater than 1 ppm by weight for Skeletonema costatum. 
Suitable imidazolines for use as corrosion inhibitors include, but are not necessarily limited to N-ethoxy, 2-substituted imidazolines, the N-ethoxy substituent comprising an amount of ethylene oxide effective to render said imidazoline water soluble, preferably from about 3 to about 9 moles of ethylene oxide, and the 2-substituent comprising an unsaturated or polyunsaturated fatty chain comprising less than about 18 carbon atoms, preferably less than about 10 carbon atoms, more preferably less than about 8 carbon atoms. Preferably, the fatty chain has at least 6 carbon atoms, most preferably from about 6 to about 8 carbon atoms.
The foregoing imidazolines are prepared by reacting a starting amine, preferably an N-substituted amine, most preferably 2,2-aminoethylamino ethanol (AEEA) or a diethylene tetramine (DETA), with a fatty acid to form an imidazoline. A most preferred starting amine is an N-substituted ethylene diamine having the formula H2NCH2CH2NHRMH, wherein R is an organic moiety and xe2x80x94MH is a terminal group comprising a hetero atom such as oxygen, nitrogen or sulfur, preferably oxygen or nitrogen, and at least one hydrogen, providing a site for attachment of ethylene oxide. Although R may include nitrogen atoms, it is preferred for R to be an alkylene, an arylene, or an aralkylene. Of these, preferred R groups are ethylene, isopropylene and xe2x80x94(CH2CH2O)n(CH2CH2)xe2x80x94, wherein n is an integer from about 1 to about 30. Out of these possibilities, preferred R groups are ethylene and the group xe2x80x94(CH2CH2O)n(CH2CH2)xe2x80x94 wherein n is an integer from about 1 to about 17. Most preferably, R is ethylene.
The group MH provides a site for attachment of ethylene oxide for ether or polyether formation. Preferably, MH is selected from the group consisting of xe2x80x94OH, xe2x80x94NH2, or xe2x80x94SH, with SH being least preferred and xe2x80x94OH being most preferred. Specific, preferred N-substituted ethylene diamines include, for example, NH2CH2CH2NHxe2x80x94CH2CH2(CH3)OH; NH2CH2CH2NHxe2x80x94CH2CH2NH2; and, most preferably, NH2CH2CH2NHxe2x80x94CH2CH2OH.
The starting amine and the fatty acid are reacted in about a 1:1 molar ratio under a vacuum with the addition of heat, such as up to about 240xc2x0 C., until all water is removed. The resulting imidazoline is then ethoxylated to build the N-substituent of the imidazoline to include a total of 3-9 moles of ethylene oxide, as necessary, to render the product water-soluble. As used herein, the term water-soluble means miscible with water at the concentration to be employed for corrosion inhibition. The resulting product has the following structure: 
wherein R and Rxe2x80x2 (the residue of the fatty acid) are alkyl groups comprising from about 6 to about 28 carbon atoms; M is t he residue from the xe2x80x94MH group after removal of the R, preferably xe2x80x94Oxe2x80x94, xe2x80x94NHxe2x80x94, or xe2x80x94Sxe2x80x94, most preferably xe2x80x94Oxe2x80x94; x (the number of xe2x80x94RM groups) is 0 or 1 and y is an integer from 0 to about 28 selected so that the total number of ethoxy units in the N-substituent is from about 1 to about 28, preferably from about 3 to about 9).
In order to be effective, the corrosion inhibitor preferably inhibits corrosion to about 50 mils per year (mpy) or less, as measured by green kettle testing. Imidazolines having 8 or fewer carbon atoms may be effective when used alone as corrosion inhibitors, but are more effective and preferably are used in combination with a wetting agent. Suitable wetting agents include, but are not necessarily limited to oxyalkylated alcohols having from 6 to about 32 carbon atoms, preferably from about 8 to about 10 carbon atoms. Oxyalkylation, preferably ethoxylation, makes the alcohol more water-soluble. Each carbon atom of the alcohol preferably should have at least one hydrogen to provide superior biodegradability. Alfol 8-10 (a mixture of C8 to C10 alcohols), which is available from a variety of sources, is especially suitable.
The alcohol may be ethoxylated using standard techniques. For example, the alcohol may be heated with a base or amine catalyst to a temperature of from about 100xc2x0 C. to about 150xc2x0 C., depending upon the catalyst, and ethylene oxide may be added thereto. The resulting ethoxylated alcohol has the structure R1Oxe2x80x94(CH2CH2O)zH, wherein R1 is a substituted or unsubstituted alkyl, aryl, or aralkyl group of from about 6 to about 32, preferably from about 8 to about 10 carbon atoms. R1 preferably is an alkyl group, most preferably an unsubstituted alkyl group. The relative proportion of ethylene oxide to alcohol depends on the degree of ethoxylation desired to provide sufficient water-solubility and biodegradability. Generally, the heavier the alcohol, the greater the degree of ethoxylation that is feasible. Although any degree of ethoxylation is feasible, economic practicalities suggest that it is not desirable to add more than about ten moles of ethylene oxide per mole of alcohol. Therefore, z preferably is an integer from about 1 to about 10, more preferably from about 2 to about 5, and most preferably from about 2 to about 3.
The corrosion inhibitor also may comprise a solvent, preferably an environmentally compatible solvent such as water, ethylene glycol, or propylene glycol. The blends have been found generally to be water-soluble; however some compositions with a low degree of ethoxylation are merely water-dispersible. In such cases, the use of isopropyl alcohol may clarify the solution, however the use of isopropyl alcohol is discouraged due to its lack of environmental compatibility. If no other components are present, the weight ratio of corrosion inhibitor to solvent is from about 2:1 to about 1:2, preferably about 1:1.
The effective composition of inhibitor actives (that is, the concentration at which corrosion inhibition is provided) is in the range of from about 1 to about 1000 ppm, preferably from about 5 to about 250 ppm, most preferably about 250 ppm. Rapid dilution of the inhibitor occurs quickly, e.g, in overboard brine from off-shore oil production.
The invention will be better understood with reference to the following examples, which are illustrative only, and should not be construed as limiting the invention to any particular embodiment.